Generating displacement and theroacoustic refrigerator

ABSTRACT

A displacement generator ( 10 ) has a housing ( 12 ) defining a chamber ( 14 ) containing an incompressible liquid ( 15 ). A port ( 22 ) of the housing ( 12 ) is closed by a movable member ( 24 ). Within the chamber ( 14 ), opposing, convex, flexible walls ( 16.1, 16.2 ) form an internal modulating chamber ( 18 ), which optionally contains a compressible gas. Opposed ends of the walls ( 16.1, 16.2 ) can be displaced toward and away from each other by a motion transducer, e.g. a stack of ceramic piezoelectric members ( 20 ), respectively to pressurize and depressurize the chamber ( 14 ) and thus to displace the member ( 24 ) to and fro to form an output of the displacement generator.

THIS INVENTION relates to a method of generating displacement, to adisplacement generator, and to a thermo-acoustic refrigerator.

U.S. Pat. No. 5,481,152 in the name of Buschulte discloses apiezoelectric actuator including a hollow body 1 formed as a round tube2 of piezoelectric material. At least one end of the tube mounts anelastic membrane 10 via an annular carrier member 8. The hollow bodyforms an enclosed chamber 13 filled with incompressible fluid 14.Operating the piezoelectric element changes the volume of the chamberand causes commensurate deformation of the elastic membrane. In anotherembodiment, elastic membranes are provided at both ends. In yet anotherembodiment, two piezoelectric tubes, one received with clearance withinthe other, form an annular chamber to provide more versatility in theamount of deformation of the elastic membrane which can be achieved.

In accordance with a first aspect of this invention, there is provided adisplacement generator which includes

a housing defining an enclosed chamber, the housing including a portexposed to the chamber;

at least one flexible wall exposed to the chamber;

a fluid contained within the chamber;

a movable member in the port which movable member is exposed to thefluid;

a motion transducer able to generate motion and being operativelyconnected to said at least one flexible wall selectively to flex said atleast one flexible wall.

In use, when the wall is flexed, the volume of the chamber is changedwhich causes movement of the fluid through the port which causescommensurate movement of the movable member which acts as a displacementoutput of the displacement generator.

In one species of embodiment, said at least one flexible wall may be anouter wall of the housing, forming part of an outer boundary of thechamber. Preferably, said at least one flexible wall may be a firstflexible wall, the outer wall of the housing providing also an opposingsecond flexible wall opposite to said first flexible wall, said firstand second flexible walls providing an opposing wall pair arranged toflex in opposing directions to increase/decrease a spacing therebetween,said opposing wall pair forming an enclosed actuating chamber which issaid enclosed chamber. Further, said fluid may be a relativelyincompressible actuating fluid, preferably a liquid, the displacementgenerator including a modulating chamber surrounding said actuatingchamber and a relatively compressible modulating fluid, preferably agas, at a predetermined pressure, within the modulating chamber, theflexible well pair having outer surfaces opposite to inner surfaceswhich are exposed to the actuating fluid, the outer surfaces beingexposed to the modulating fluid. Appropriate selection of the actuatingfluid and the modulating fluid in conjunction with the physicalconfiguration of the device will then allow a desired natural frequencyof the device to be obtained.

In another species of embodiment, said at least one flexible wall may bean inner wall within the housing, said housing being an outer housingdefining the enclosed chamber, said at least one flexible wall formingan internal wall within the enclosed chamber. Preferably, said at leastone flexible wall may be a first flexible wall, the displacementgenerator including also an opposing second flexible wall opposite tosaid first flexible wall, said first and second flexible walls providingan opposing wall pair arranged to flex in opposing directions toincrease/decrease a spacing therebetween, said opposing wall pairforming an enclosed modulating chamber which is an inner chamber withinsaid enclosed chamber, which is an actuating chamber. Further, saidfluid in said actuating chamber may be relatively incompressible, e.g. aliquid, the displacement generator including a modulating fluid e.g. agas, at a predetermined pressure, which is relatively compressible insaid modulating chamber.

Said first and second flexible walls of said flexible wall pair mayadvantageously be curved, said motion transducer being arranged todisplace opposed ends of the flexible wall pair relative to each otherto change the curvature of the respective walls thus to cause arelatively large change in volume of the actuating chamber in responseto a relatively small displacement of said opposing ends. Thus themotion transducer may be connected in between said opposed ends, themotion transducer being selectively extensible/contractible.

The motion transducer may be selected from the group consisting of anelectrostriction device, a magnetostriction device, and a piezoelectricdevice, Preferably the motion transducer may be in the form of apiezoelectrical ceramic stack extending between the opposed ends.

In accordance with a second aspect of this invention, there is provideda method of generating displacement by a displacement generator whichincludes

a housing defining an enclosed chamber, the housing including a portexposed to the chamber;

at least one flexible wall exposed to the chamber;

a fluid contained within the chamber;

a movable member in the port which movable member is exposed to thefluid,

the method including actuating a motion transducer which is operativelyconnected to said at least one flexible wall selectively to flex said atleast one flexible wall to change the volume of the chamber to move themovable member.

When said at least one flexible wall is an outer wall of the housing andforms part of an outer boundary of the chamber, and when said at leastone flexible wall is a first flexible well, the outer wall of thehousing providing also an opposing second flexible wall opposite to saidfirst flexible wall, said first and second flexible walls providing anopposing wall pair forming an enclosed actuating chamber which is saidenclosed chamber, the method may include selectively flexing, by meansof said motion transducer, said opposing wall pair in opposingdirections to increase/decrease a spacing between the first and secondflexible walls.

When said fluid is a relatively incompressible actuating fluid, and thedisplacement generator includes a modulating chamber surrounding saidactuating chamber and a relatively compressible modulating fluid withinthe modulating chamber, the flexible wall pair having inner surfaceswhich are exposed to the actuating fluid and outer surfaces opposite tothe inner surfaces, the outer surfaces being exposed to the modulatingfluid, the method may include modulating the change in volume of theactuating chamber by resiliently resisting said change in volume bymeans of the modulating fluid.

When said at least one flexible wall is an inner wall within thehousing, said housing being an outer housing wall defining the enclosedchamber, and when said at least one flexible wall is a first flexiblewall, the displacement generator including also an opposing secondflexible wall opposite to said first flexible wall, said first andsecond flexible walls providing an opposing wall pair internally withinthe enclosed chamber, the opposing well pair being arranged to flax inopposing directions to increase/decrease a spacing therebetween, saidopposing wall pair forming an enclosed modulating chamber which is aninner chamber within said enclosed chamber, the enclosed chamber beingan actuating chamber, said fluid in said actuating chamber beingrelatively incompressible, the displacement generator including amodulating fluid which is relatively compressible in said modulatingchamber, the method may include modulating the change in volume of theactuating chamber by resiliently resisting said change in volume bymeans of the modulating fluid.

When said first and second flexible walls of said flexible wall pair arecurved, the method may include displacing, by means of said motiontransducer, opposed ends of the flexible wall pair relative to eachother to change the curvature of the respective walls thus to cause arelatively large change in volume of the actuating chamber in responseto a relatively small displacement of said opposing ends.

The motion transducer may be connected in between said opposed ends,actuation of the motion transducer being selectively extending orcontracting the motion transducer.

In accordance with a third aspect of this invention, there is provided athermo-acoustic refrigerator including a resonator and a displacementgenerator drivingly connected to the resonator, the displacementgenerator being in accordance with the first aspect of this invention.

The invention is now described by way of examples with reference to theaccompanying diagrammatic drawings. In the drawings

FIGS. 1, 2 and 3 show, schematically, in section, three embodiments ofdisplacement generators in accordance with the Invention; and

FIG. 4 shows, in axial section, fragmentarily, a thermo-acousticrefrigerator in accordance with the invention.

With reference to FIG. 1 of the drawings, a first embodiment of adisplacement generator in accordance with the invention is generallyindicated by reference numeral 10. It is emphasized that thedisplacement generator 10 is shown schematically to explain theprinciples of the invention.

The displacement generator 10 includes a housing 12 defining a chamber14. The chamber 14 is an actuating chamber and is filled with arelatively incompressible fluid, preferably a liquid, generallyindicated by reference numeral 15.

Within the chamber 14, there is provided a pair of opposing flexiblewalls 16.1 and 16.2 which form internal walls of the housing 12. Theflexible walls 16.1 and 16.2 are curved, and oppose each other to form aflexible wall pair in the form of a convex closed figure forming amodulating chamber 18 therebetween.

A ceramic stack 20 in the form of at least one, and in practice aplurality of, elongate piezoelectric members, is connected at endsthereof to and extend between opposing ends of the flexible wall pair16.1, 16.2. The ends of the stack 20 are connected to ends of the wallpair such that contraction of the stack will cause bulging of theflexible wall pair i.e. will increase the volume of the modulatingchamber 18 and will thus decrease the volume of and increase thepressure within the actuating chamber 14. Conversely, extension of theceramic stack 20 will flatten the flexible wall pair causing the chamber18 to decrease in volume and will increase the volume of and decreasethe pressure in the actuating chamber 14.

The housing 12 defines a port 22 which is closed by means of adisplaceable member 24, which may conveniently be in the form of aplunger disc and which is movable along the port 22.

Thus, when the ceramic stack 20 is contracted to bulge to increase thechamber 18 and to increase the pressure in the chamber 14, the liquid 15flows outwardly through the port 22 and displaces the plunger disc 24outwardly. Conversely, extension of the ceramic stack 20 flattens theflexible wall pair, decreases the chamber 18, decreases the pressurewithin the housing 12 and causes the liquid 15 to flow inwardly alongthe port 22 thus causing ambient pressure externally on the plunger disc24 to move that inwardly in sympathy with inward movement of the liquid15.

With reference to FIG. 2, a second embodiment of a displacementgenerator in accordance with this invention is generally indicated byreference numeral 110, The displacement generator 110 is in manyrespects similar to the displacement generator 10 of FIG. 1 and likereference numerals refer to like features or components. The FIG. 2embodiment is not described in detail, and only one difference will beemphasized.

The flexible wall pair 116.1 and 116.2 provides an externally concavebody. Thus, in this embodiment, when the ceramic stack 120 contracts, itincreases the curvature of the flexible walls 116.1 and 116.2 thuscauses the chamber 118 to decrease in volume, which causes the chamber114 to increase in volume and the pressure to decrease, thus causingreturn flow of the liquid 115 through the port 122 and thus inwardmovement of the plunger disc 124.

Conversely, if the ceramic stack 120 extends, the flexible walls 116.1and 116.2 will flatten causing an increase in the volume of the chamber118 thus causing flow of liquid 115 outwardly in the port 122.

Thus, in the embodiments of FIGS. 1 and 2, contraction of thepiezoelectric stacks cause respectively opposite displacements of theplunger disc.

With reference to FIG. 3, a third embodiment of a displacement generatorin accordance with the invention is generally indicated by referencenumeral 210. Also the embodiment 210 is similar in many respects to theembodiments of FIGS. 1 and 2 and again like reference numerals are usedto denote like components and features. Again the embodiment of FIG. 3is not described in detail and a single difference is merely emphasized.

In the embodiment of FIG. 3, the chamber within the housing 212 is infact the modulating chamber 218, and the chamber enclosed between theopposing flexible walls 216.1 and 216.2 is the actuating chamber 214filled with incompressible liquid 215. The port 222 is in communicationwith the actuating chamber 214. Thus, in the embodiment of FIG. 3,contraction of the ceramic stack 220 causes an increase in curvature ofthe flexible walls 216.1 and 216.2 thus decreasing the volume of thechamber 214 and forcing the liquid 215 outwardly in the port 222 thusdisplacing the displacement member or plunger 224 outwardly.

Conversely, extension of the ceramic stack 220 flattens the flexiblewalls 216.1 and 216.2 to increase the chamber 214 and to cause pressureexternally of the displacement disc 224 to move it and the liquid 215inwardly in the port 222.

Another important aspect of this invention is described with referenceto FIG. 3, but it can also be applied to the embodiments of FIGS. 1 and2.

The modulating chamber 218 is filled with a compressible gas 219 whichwill tend to counteract the effect of contraction and extension of theceramic stack 220. Thus, if the ceramic stack 220 is extended toincrease the volume of the chamber 214, that will decrease the volume ofthe chamber 218 which will pressurize the compressible gas 219. It is tobe appreciated that the ratio of compression, in the embodiment of FIG.3, will be relatively small bearing in mind that the modulating chamber218 is probably much larger than the actuating chamber 214. Furthermore,it is to be appreciated that the gas 219 is compressible.

When a modulating gas is used with the embodiments of FIGS. 1 and 2, thegas will be contained within the flexible wall pair.

The pressure of the modulating gas is selected in conjunction with thenominal pressure to which the displaceable member is subjected suchthat, when the device is inoperative but connected to a system it isintended to drive, no or little residual stress is present in thedriving system. Thus, the stresses and strains in the driving systemwill be due to, mostly or exclusively, operation of the driving systemitself, as opposed to subjecting the driving system to residual stressand strain and superimposing thereon cyclic stresses and strains due toits operation. Conveniently, the gas can be exposed via a small bleedaperture or orifice to a refrigerator gas of a refrigerator as will bedescribed below, to equalize presures. In use, the bleed aperture is toosmall to allow instantaneous equalizing in accordance with pressurefluctuations when the device is in operation.

During designing the displacement generator, a number of options existto design to a desired natural frequency, for example by selecting thevolume of the chamber, the curvature of the flexible wall, and the like.At least some of these may be adjusted to adjust the natural frequency.

It is to be appreciated that a displacement generator can be designed toa specific natural frequency, as mentioned above. In practice, theactual natural frequency may differ from the desired natural frequency.

The development mentioned above, has the advantage of being able toselect the relatively incompressible fluid, preferably liquid, and therelatively compressible fluid, preferably gas, in respect of the theirphysical properties, and also, in the case of the compressible gas, thepressure with which it is present within the modulating chamber 218.Thus, a displacement generator in accordance with the invention can inthis fashion be tuned in respect of its natural frequency. This featureis of particular importance if a specific natural frequency or a naturalfrequency within a narrow range, is required.

With reference to FIG. 4, a thermo-acoustic refrigerator in accordancewith a third aspect of this invention is generally indicated byreference numeral 30.

The thermo-acoustic refrigerator 30 comprises, generally, a resonator 32which is connected to, such as to be driven by, a displacement generatorin accordance with the second aspect of the invention and which isgenerally indicated by reference numeral 10. Thus, the resonator 32 isinternally exposed to the external side of the displaceable member ofplunger disc 24. For convenience, reference is made to a displacementgenerator 10 and to a plunger disc 24. It is emphasized that thedisplacement generator can be any suitable displacement generator inaccordance with this invention, for example, any of the displacementgenerators of FIGS. 1, 2 and 3.

The thermo-acoustic refrigerator further has a hot side reducer 34, ahot side heat exchanger 35, a stack 36 and a cold side heat exchanger 38in conventional fashion.

The thermo-acoustic refrigerator is operated by selectively applyingalternating voltage to the ceramic stack of the displacement generator10 to vibrate the plunger disc 24 and thus to drive the resonator 32. Inthis regard, it is to be appreciated that the resonator 32 will have anatural frequency and the natural frequency of the displacementgenerator is preferably tuned, as described above, to the naturalfrequency of the resonator 32.

It is a well recognized requirement of thermo-acoustic refrigeratorsthat an electric acoustic transducer or loudspeaker which isconventionally used to drive a thermo-acoustic refrigerator, must beable to drive a high load and that conventional electrodynamictransducers are often unable to generate sufficient displacement underthese load conditions. The Applicant is aware that a large amount ofeffort and funds have been invested in developing electro-acoustictransducers or loudspeakers to satisfy these requirements.

The Applicant contends that this invention is a teaching away fromconventional thinking in respect of driving a thermo-acousticrefrigerator by piezoelectricity. It is well known that apiezoelectric-effect, for example in a conventional ceramicpiezoelectric member as used in the extensional mode, is generally ofhigh force but very small displacement, the displacement being in themicron range rather than in the millimetre range. It is thus veryimportant that, in accordance with this invention, a displacementgenerator is provided to amplify a small displacement of high force. Itis also regarded as very important that the displacement generator inaccordance with this invention operates on the basis of causing arelatively large change in volume by means of a relatively small onedimensional change i.e. a change in length. It is further very importantthat the change in volume is transmitted very effectively to a membranewhich can vibrate. It is yet further of importance that the naturalfrequency of the system can be tuned.

The Inventors believe that this invention improves on other systems inrespect of the force-displacement characteristics the displacementgenerator in accordance of this invention can achieve.

Thus, this invention provides a simple, elegant and effective drive fora thermo-acoustic refrigerator and improves, especially in respect ofits force/displacement characteristics, on known systems.

What is claimed is:
 1. A displacement generator (10, 110, 210) whichincludes a housing (12, 112, 212) defining an enclosed chamber (14, 114,214), the housing including a port (22, 122, 222) exposed to thechamber; at least one flexible wall (16.1, 16.2; 116.1, 116.2; 216.1,216.2) exposed to the chamber and being flexible in a general flexdirection; a fluid (15, 115, 215) contained within the chamber; amovable member (24, 124, 224) in the port which movable member isexposed to the fluid; a motion transducer (20, 120, 220) able togenerate motion in a motion transducer direction and being operativelyconnected to said at least one flexible wall selectively to flex said atleast one flexible wall, characterized in that the motion transducer isseparate and apart from said flexible wall other than said operativeconnection to said flexible wall and in that said general flex directionis transverse to said motion transducer direction.
 2. A displacementgenerator as claimed in claim 1 in which said at least one flexible wallis an outer wall of the housing and forms part of an outer boundary ofthe chamber.
 3. A displacement generator as claimed in claim 2 in whichsaid at least one flexible wall is a first flexible wall, the outer wallof the housing providing also an opposing second flexible wall oppositeto said first flexible wall, said first and second flexible wallsproviding an opposing wall pair arranged to flex in opposing directionsto increase/decrease a spacing therebetween, said opposing wall pairforming an enclosed actuating chamber which is said enclosed chamber. 4.A method as claimed in claim 3 in which said fluid is a relativelyincompressible actuating fluid, the displacement generator including amodulating chamber surrounding said actuating chamber and a relativelycompressible modulating fluid within the modulating chamber, theflexible wall pair having outer surfaces opposite to inner surfaceswhich are exposed to the actuating fluid, the outer surfaces beingexposed to the modulating fluid.
 5. A displacement generator as claimedin claim 1 in which said at least one flexible wall is an inner wallwithin the housing, said housing being an outer housing defining theenclosed chamber, said at least one flexible wall forming an internalwall within the enclosed chamber.
 6. A displacement generator as claimedin claim 5 in which said at least one flexible wall is a first flexiblewall, the displacement generator including also an opposing secondflexible wall opposite to said first flexible wall, said first andsecond flexible walls providing an opposing wall pair arranged to flexin opposing directions to increase/decrease a spacing therebetween, saidopposing wall pair forming an enclosed modulating chamber which is aninner chamber within said enclosed chamber, which is an actuatingchamber.
 7. A displacement generator as claimed in claim 6 in which saidfluid in said actuating chamber is relatively incompressible, thedisplacement generator including a modulating fluid which is relativelycompressible in said modulating chamber.
 8. A thermo-acousticrefrigerator including a resonator and a displacement generatordrivingly connected to the resonator, the displacement generator beingin accordance with claim 4 or claim 7, in which a resonant frequency ofthe displacement generator has been tuned to a resonent frequency of theresonator by suitable selection of at least one of the modulating fluidand a pressure of the modulating fluid in the modulating chamber.
 9. Adisplacement generator as claimed in claim 3 or claim 6 in which saidfirst and second flexible walls of said flexible wall pair are curved,said motion transducer being arranged to displace opposed ends of theflexible wall pair relative to each other to change the curvature of therespective walls thus to cause a relatively large change in volume ofthe actuating chamber in response to a relatively small displacement ofsaid opposing ends.
 10. A displacement generator as claimed in claim 9in which the motion transducer is connected in between said opposedends, the motion transducer being selectively extensible/contractible.11. A displacement generator as claimed in claim 10 in which the motiontransducer is selected from the group consisting of an electrostrictiondevice, a magnetostriction device, and a piezoelectric device.
 12. Adisplacement generator as claimed in claim 10 in which the motiontransducer is in the form of a piezoelectrical ceramic stack extendingbetween the opposed ends.
 13. A thermo-acoustic refrigerator including aresonator and a displacement generator drivingly connected to theresonator, the displacement generator being in accordance with claim 10.14. A thermo-acoustic refrigerator including a resonator and adisplacement generator drivingly connected to the resonator, thedisplacement generator being in accordance with claim 3 or claim
 6. 15.A method of generating displacement by a displacement generator (10,110, 210) which includes a housing (12, 112, 212) defining an enclosedchamber (14, 114, 214), the housing including a port (22, 122, 222)exposed to the chamber; at least one flexible wall (16.1, 16.2; 116.1,116.2; 216.1, 216.2) exposed to the chamber and being flexible in ageneral flex direction; a fluid (15, 115, 215) contained within thechamber; a movable member (24, 124, 224) in the port which movablemember is exposed to the fluid; a motion transducer (20, 120, 220) whichis operatively connected to said flexible wall and which is separate andapart from said flexible wall other than said operative connection tosaid flexible wall; the method being characterized by actuating saidmotion transducer to move in a motion transducer direction transverse tosaid general flex direction selectively to flex said at least oneflexible wall to change the volume of the chamber to move the movablemember.
 16. A method as claimed in claim 15 in which said at least oneflexible wall is an outer wall of the housing and forms part of an outerboundary of the chamber, in which said at least one flexible wall is afirst flexible wall, the outer wall of the housing providing also anopposing second flexible wall opposite to said first flexible wall, saidfirst and second flexible walls providing an opposing wall pair formingan enclosed actuating chamber which is said enclosed chamber, in whichmethod the motion transducer selectively flexes said opposing wall pairin opposing directions to increase/decrease a spacing between the firstand second flexible walls.
 17. A method as claimed in claim 16, in whichsaid fluid is a relatively incompressible actuating fluid, thedisplacement generator including a modulating chamber surrounding saidactuating chamber and a relatively compressible modulating fluid withinthe modulating chamber, the flexible wall pair having inner surfaceswhich are exposed to the actuating fluid and outer surfaces opposite tothe inner surfaces, the outer surfaces being exposed to the modulatingfluid, the method including modulating the change in volume of theactuating chamber by resiliently resisting said change in volume bymeans of the modulating fluid.
 18. A method as claimed in claim 15, inwhich said at least one flexible wall is an inner wall within thehousing, said housing being an outer housing wall defining the enclosedchamber, said at least one flexible wall being a first flexible wall,the displacement generator including also an opposing second flexiblewall opposite to said first flexible wall, said first and secondflexible walls providing an opposing wall pair internally within theenclosed chamber, the opposing wall pair being arranged to flex inopposing directions to increase decrease a spacing therebetween, saidopposing wall pair forming an enclosed modulating chamber which is aninner chamber within said enclosed chamber, the enclosed chamber beingan actuating chamber, said fluid in said actuating chamber beingrelatively incompressible, the displacement generator including amodulating fluid which is relatively compressible in said modulatingchamber, the method including modulating the change in volume of theactuating chamber by resiliently resisting said change in volume bymeans of the modulating fluid.
 19. A method as claimed in claim 17 orclaim 18 in which said first and second flexible walls of said flexiblewall pair are curved, the method including displacing, by means of saidmotion transducer, opposed ends of the flexible wall pair relative toeach other to change the curvature of the respective walls thus to causea relatively large change in volume of the actuating chamber in responseto a relatively small displacement of said opposing ends.
 20. A methodas claimed in claim 19 in which the motion transducer is connected inbetween said opposed ends, actuation of the motion transducer beingselectively extending or contracting the motion transducer.